Active noise cancellation (ANC) is a sound reduction method which detects undesired sounds and generates a negative copy of such sounds (anti-sound). An anti-sound is a signal of the same frequency and amplitude as the detected undesired sound, but of opposite sign. The undesired sound and outputted anti-sound destructively interfere with one another, resulting in significant noise reduction.
Most ANC systems aim to create a quiet zone around a specific spot (the listener) by suppressing unwanted sounds from a remote source or surrounding environment. Such ANC systems thus require placing the audio input and output elements in close proximity to the listener. Another approach is to place the ANC device at the noise source. This method has the advantage of reducing noise in a wide area, but only for a specific source.
For example, U.S. Pat. No. 8,571,227 describes an ANC earphone having an acoustic path including a cavity and a pipe leading to the auditory canal which are arranged to form an oscillator which in use has the effect of recovering the open loop system phase characteristics at a selected frequency or frequency range. The earphone also has two parts which can be adjusted relative to each other to allow the earphone to be comfortably and correctly positioned in use.
U.S. patent publication Nos. 2014/0169580 describes an active noise cancellation system provided in a head support to cancel noise for a supine user, lying on a bed.
U.S. Pat. No. 5,844,996 describes a system for attenuating noise which can be sensed by the auditory nerve comprising a microphone positioned in a first sound region for sensing noise present in the first sound region for creating a first electrical signal having a frequency and amplitude corresponding to the noise sensed in the first sound region. Circuitry is provided for processing the first electrical signal to create a second electrical signal having the same frequency but of opposite amplitude and sign of the first electrical signal. A speaker is provided for converting the second electrical signal to anti-noise for attenuation of the sensed noise in a second sound region spaced from the first sound region. A microphone is disposed in the second sound region for detecting any noise above a predetermined noise level in the second sound region to provide an error correction signal. Circuitry is provided responsive to the error correction signal to modify the second electrical signal to bring the noise in the second sound region so that it is below a predetermined noise level.
U.S. Pat. No. 8,325,934 describes an electronic pillow including a pillow unit encasing at least one error microphone and at least one loudspeaker in electrical connection with a controller unit, the pillow unit also including a power source, and a reference sensing unit including at least one reference microphone in electrical connection with the controller unit, the controller unit including an algorithm for controlling interactions between the error microphone, loudspeaker, and reference microphone. Unwanted noise is abated by detecting an unwanted noise with a reference microphone, analyzing the unwanted noise, producing an anti-noise corresponding to the unwanted noise in a pillow, and abating the unwanted noise.
U.S. Pat. No. 7,317,801 describes an active acoustic noise reduction system which comprises a single input transducer and an output actuator that are physically located next to each other in the same location. In one embodiment, the input transducer and the output actuator are a hybrid represented by a single element. The active noise reduction system is located as close as possible to the noise source and functions to generate an anti-noise cancellation sound wave with minimum delay and opposite phase with respect to the noise source. The noise reduction system also comprises a non-linearity correction circuit, a delayed cancellation circuit and variable gain amplifier. The system provides user control of the quiet zones generated by the system by varying the gain of the variable gain amplifier.
In spite of these developments there is a great need for further improvement in the ANC field, particularly for improving a user's comfort and improving noise reduction levels.